1. Field of the Invention
The present invention relates to a fine pattern forming method used in a semiconductor device fabricating process, and more specifically to a fine pattern forming method that is capable of precisely adjusting a pattern dimension when forming a pattern having a dimension below a photolithography resolution limit without increasing production cost.
2. Description of the Related Art
Along with a demand for higher integration, a line width and a separation width required for a fabrication process become smaller. Generally, a fine pattern is formed by employing a photolithography technology in order to create a photoresist pattern and etching various underlying thin films using the photoresist pattern as an etching mask. Therefore, the photolithography technology is playing a crucial role when forming a fine pattern. However, further decreasing dimensions in semiconductor devices demand finer patterns below the resolution limit of the photolithography technology. Specifically, because it is thought that ArF liquid immersion photolithography, which is a main-stream photolithography technology of today, may reach a critical limit in a 4×nm generation, a new fine pattern forming technology called double patterning, which is promising to realize a finer pattern in a 3×nm generation, is under vigorous research and development.
Such an enabling technology for forming a pattern below the resolution limit is disclosed, for example, in Patent Document 1.
Patent Document 1 discloses a fine pattern forming method for forming a fine pattern in the following manner. First, a first photosensitive film pattern (a first pattern) having first lines is formed; the first pattern is baked; a silicon oxide film is formed on the first pattern; a second photosensitive film pattern (a second pattern) is formed having second lines between every adjacent two of the first lines; the silicon oxide film is etched with the second pattern; and a thin film below the first pattern is etched with the first and the second patterns.
According to this method disclosed in Patent Document 1, because two photomasks are used to form the first and the second patterns, the effective resolution can be more than doubled, when compared with a fine pattern formed with only one photomask, thereby enabling the fine pattern below the resolution limit of the current photolithography technology.
As a fine pattern forming method for forming a pattern below the resolution limit, which includes the above-mentioned method, a side wall transfer (SWT) process, a Litho-Etch-Litho-Etch (LELE) process and a Litho-Litho-Etch (LLE) process attract attention. In the LELE process, after the first lithography is carried out to form a first pattern, a first etching process is carried out using the first pattern; and then after second lithography is carried out to form a second pattern, a second etching process is carried out using the second pattern. On the other hand, in the LLE process, after the first lithography is carried out, the second photolithography is carried out and then an etching process is carried out. Namely, the LLE process has only one etching process while both the lithography and the etching are repeated twice in the LELE process. Therefore, the LLE process is advantageous in reducing production cost, compared with the LELE process.
However, in the LLE process, when the second photoresist film to be used in forming the second pattern is coated on the first pattern formed on a substrate, the first pattern has to be protected, or undergo a so-called resist freezing process, in order to prevent the first pattern from being dissolved by a solution such as an organic solvent included in the second photoresist film. As a technology for protecting the first pattern, a silicon oxide film is deposited on the first pattern as a protecting film, an example of which is disclosed in Patent Document 2.
Specifically, Patent Document 2 discloses a technology capable of preventing the photoresist pattern from being thinned and reducing striation and/or wiggling in a fine pattern by forming the silicon oxide film on the photoresist film, although Patent Document 2 does not describe a method for forming a fine pattern below the resolution limit.    Patent Document 1: Japanese Patent Publication No. 2757983.    Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2004-080033.